[Rockhounds] The world's largest crystal cave

[linda at middleearthminerals.com]

Alan, I think that's essentially correct except that I've seen other
minerals (zeolites, for example) forming the "rind" although of course,
quartz (including chalcedony and agate) is the most common rind.  Sometimes,
especially for sedimentary geodes, the quartz rind is diffuse, permeating
the sandstone (or other matrix material) strongly enough to take a polish.
And occasionally the rind is a cast.  The polyhedra from Brazil and
elsewhere are thought to be quartz pseudomorphing calcite, and they result
in angular "geodes" which some people may reject as geodes but any
mathematician would recognize as topologically equivalent!

The mindat definition of geode includes this:
"Geodes differ from vugs in that geodes can be separated free from any
matrix, because the walls of geodes are strong enough to maintain the
integrity of their initial shapes."

To me, that would include what some people might call a vug if the vug is
encrusted thickly enough everywhere to allow for its intact separation from
the matrix.  The judges at the San Diego Fair were willing to admit geodes
with rinds of various minerals as well as those of angular shapes.  Of
course, they may have been pandering somewhat to the public's taste because
the broader the definition, the better the display.

Linda

-----Original Message-----
From: Rockhounds <rockhounds-bounces at rockhounds.drizzle.com> On Behalf Of
Alan Goldstein
Sent: Tuesday, July 26, 2022 4:59 PM
To: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
<rockhounds at rockhounds.drizzle.com>
Subject: Re: [Rockhounds] The world's largest crystal cave

I realize that like the word "theory," geode has multiple definitions. I've
read that - in geological terms - a geode is quartz-lined somewhat spherical
rock within sedimentary or igneous rocks and a "vug" is a hollow pocket
lines with crystals (no quartz rind). With that in mind, the beautiful
calcite-lined cavity in Spain is a vug. What have you read about the
difference between a geode and vug?

Alan G.
________________________________
From: Rockhounds <rockhounds-bounces at rockhounds.drizzle.com> on behalf of
Kreigh Tomaszewski <kreigh at gmail.com>
Sent: Monday, July 25, 2022 8:51 AM
To: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
<rockhounds at rockhounds.drizzle.com>
Subject: [Rockhounds] The world's largest crystal cave

In abandoned silver mine in Pulpí, in Spain's south-eastern Almería
Province, lies a treasure that's not made from any precious metal at all.
Instead, what's hidden here is the world's largest geode – a natural crystal
phenomenon that has stunned scientists.

As Mila Carretero, geologist and coordinator of the Pulpí Geode, explained,
a geode is a cavity inside a rock that is covered with crystals. Sitting
against a backdrop of oversized crystal spars, she broke open a tiny rock
with tiny gems inside, to show a comparison. "It's the same as the one I
have behind me, only this one is a super-sized version," she said with a
laugh as she pointed over her shoulder.

The Pulpí Geode is eight metres wide, two metres high and two metres deep.
"When it comes to a geode, by definition, this is the biggest ever
discovery," she noted, adding that Pulpí is not to be confused with another
crystal marvel, the Naica Mine in Mexico, which has larger spars (15m long
compared to Pulpí's two metres), but which is a cave lined with crystals
rather than a geode.

The geode here in Spain was originally spotted by miners in the Mina Rica, a
silver mine which operated from 1873 to 1969. But it wasn't until years
later, in 1999, that geologists found it again and brought it to the world's
attention.

"When [the original miners] blasted this rock and found a geode, they
probably got upset because they didn't like finding these crystals," said
Carretero. "It meant extra work to get rid of them. They weigh a lot and
were not profitable."

Though scientists are still researching it, they believe the whole area was
once underwater. At a certain point, volcanic activity fractured sedimentary
rocks and filled them with hot fluids. When the fluids cooled, the crystals
started forming.


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